1. Field of the Invention
This invention relates to a method of adjusting image processing conditions for use in the processing of the image signals for reproduction of a visible image which are detected from a recording medium such as a stimulable phosphor sheet carrying a radiation image recorded thereon by limitation of the irradiation field of the radiation.
2. Description of the Prior Art
When certain kinds of phosphors are exposed to radiation such as X-rays, .alpha.-rays, .beta.-rays, .gamma.-rays, cathode rays or ultraviolet rays, they store a part of the energy of the radiation. Then, when the phosphor which has been exposed to the radiation is exposed to stimulating rays such as visible light, light is emitted by the phosphor in proportion to the stored energy of the radiation. A phosphor exhibiting such properties is referred to as a stimulable phosphor.
As disclosed in U.S. Pat. No. 4,258,264 and Japanese Unexamined Patent Publication No. 56(1981)-11395, it has been proposed to use a stimulable phosphor in a radiation image recording and reproducing system. Specifically, a sheet provided with a layer of the stimulable phosphor (hereinafter referred to as a stimulable phosphor sheet) is first exposed to radiation passing through an object such as the human body to have a radiation image of the object stored thereon, and is then exposed to stimulating rays such as a laser beam which cause the stimulable phosphor sheet to emit light in proportion to the stored radiation energy. The light emitted by the stimulable phosphor sheet upon stimulation thereof is photoelectrically detected to obtain final read-out image signals for use in reproduction of a visible image, image processing is carried out on the final read-out image signals, and the radiation image of the object is reproduced as a visible image by use of the processed final read-out image signals on a recording material such as a photographic film, a display device such as a cathode ray tube (CRT), or the like.
In this specification, detection of the image signals for use in reproduction of the visible image is referred to as the final read-out, and the image signals obtained by the final read-out are referred to as the final read-out image signals. The terms "final read-out" and "final read-out image signals" are used in the same senses also in the case where the recording medium is of the type different from the stimulable phosphor sheet.
In general, the image processing in the aforesaid radiation image recording and reproducing system is carried out by use of image processing conditions adjusted for each image in accordance with the image recording portion of the object and/or the image recording method to obtain a visible image suitable for viewing, particularly for diagnostic purposes. However, it may be considered that the image processing conditions be adjusted on the basis of, for example, a histogram of the final read-out image signals (i.e. a histogram of the levels of the image signals), instead of the image recording portion of the object and/or the image recording method, or on the basis of both the final read-out image signals and the image recording portion of the object and/or the image recording method.
In the case where the image processing is carried out by use of the image processing conditions adjusted on the basis of the histogram of the final read-out image signals which directly represent the nature of each radiation image, it becomes possible to obtain a visible image always having an improved image quality, particularly a high diagnostic efficiency and accuracy, i.e. a visible image wherein the necessary object image information is always expressed within the density range suitable for viewing, particularly for diagnostic purposes, regardless of any fluctuation in the level of the radiation energy stored on the stimulable phosphor sheet, which is caused by changes in the object, the image recording portion thereof, radiation dose, or the like.
On the other hand, in the case where the aforesaid radiation image recording and reproducing system is used for medical diagnosis, and generally in the case where a radiation image of the human body is recorded, portions of the human body not related to diagnosis should not be exposed to the radiation since it is harmful to the human body. Further, when the human body portions not related to diagnosis are exposed to radiation, the radiation is scattered by such portions to the portion related to the diagnosis, and the contrast and resolution are adversely affected by the scattered radiation. Therefore, the irradiation field is often limited in the course of the recording of a radiation image.
However, in the case where a radiation image is recorded by limiting the irradiation field as mentioned above, it is not always possible to obtain such appropriate image processing conditions as originally expected to be obtained by the adjustment based on the histogram of the final read-out image signals, even though the image processing conditions are adjusted on the basis of the histogram of the final read-out image signals detected from the stimulable phosphor sheet.
This is because, in the case where image recording is carried out by limitation of the irradiation field as mentioned above, the radiation scattered by the object within the irradiation field normally passes outside of the irradiation field. The scattered radiation is absorbed and stored on the stimulable phosphor sheet which exhibits high sensitivity, and therefore the histogram of the levels of the final read-out image signals includes the image signal level caused by the scattered radiation. Accordingly, in the case where the image processing conditions are adjusted on the basis of the histogram of the final read-out image signals, it follows that the image processing conditions are adjusted based on the histogram including image signals (noise) caused by the scattered radiation, instead of the histogram of the image signals representing the true image inside of the irradiation field.
Therefore, in the case where image recording is carried out by limiting the irradiation field and the image processing conditions are to be adjusted based on the histogram of the final read-out image signals as mentioned above, the image processing conditions should preferably be adjusted based on the histogram of noise-free image signals obtained by eliminating the information caused by the scattered radiation, instead of being adjusted directly based on the histogram of the image signals detected by the final read-out.
Besides the case of image recording utilizing the stimulable phosphor sheet, the aforesaid problems with regard to the adjustment of the image processing conditions arise generally in the case where a radiation image is recorded on a recording medium by limitation of the irradiation field.
By the term "image processing conditions" as used herein are meant various conditions affecting the relationship between the input to the image processing means and the output thereof, for example, gradation processing conditions and frequency response processing conditions.